GRADING AND SCREENING 237 Shape of Rock Fragments in Nature.—By virtue of the pressure of mountain forming agencies more or less perfectly developed series of conjugate fracture planes are often found in nature and consequently the tetrahedral form is common. Incipient fracture planes exist in all rocks which have been subjected to pressure and with gravity are among the principal agents of erosion. Figure 12 shows a suite of the fragments of the principal rocks or rock forming minerals of a FIG. 12.—Tetrahedral rock fragments. Colorado mining district selected for the perfection of the tetrahedral form. All are reproduced full size. A is a fragment of pegmatite; B of vein quartz; C pinkish feldspar; D is schist and E granite. Where the country rock is bedded one point of the tetrahedron is often truncated and as one set of the conjugate planes will almost invariably favor the bedding planes the result of weathering of such a rock as schist is the production of fragments of flat shapes. Fragments such as D are difficult to find. In the district from where this came the schist is soft and most of the eroded fragments are rounded. The granite specimen came from a large boulder much broken up by fracture planes and showing tetrahedrons from 1 to 2 ft. thick to hand specimens. Shape of Fragments as Affecting Screening Work.—Of fragments resulting from massive material those approaching roundish or other isometric shapes should screen the easiest for the reason that if they get over or into the apertures of a screen any direction axis will allow them to pass if they are undersize grains. The tetrahedral grains will give more trouble since they will usually have one position which is more stable than any other and in most forms of screening apparatus they will tend to ride through more in this position than any other. The side on which the fragments usually ride through the screening apparatus is the one of largest or nearly largest area.